Typical strut-type synchronizers for manual automotive transmissions include a “shifting sleeve carrier” or “hub” that rotates with a given shaft either adjacent to a single gear, or in between a pair of gears, that are respectively mounted on the shaft for free-rotation relative to the shaft. The hub includes a set of peripheral spline teeth and several peripheral axial slots or pockets, each pocket supporting a respective strut for axial movement relative to the pocket. An annular shifting sleeve, encircling and in splined engagement with the hub, engages each strut through a spring-loaded detent that typically includes a circumferential detent groove defined on the inner periphery of the sleeve.
The detent is generally designed to maximally transmit, to the strut, a relatively small fraction of an axial shifting load that is applied to the sleeve through a suitable combination of detent spring rate, the “ramp angle” defined by the detent groove's walls or “ramps,” as measured relative to a reference line that is parallel to the rotational axis of the shaft, and the coefficient of friction between the detent and the sleeve (and, in the case of a ball-type detent, the coefficient of friction between the detent ball and the struts' ball-receiving passage). The maximum axial detent load on the strut is generally known as the synchronizer's “breakthrough load” or “BTL.” Through known approaches, transmissions intended to provide relatively “smooth” shifts typically employ synchronizer detents that achieve breakthrough loads in the range of about 40 N to about 50 N, or perhaps as little as about 5 percent of the typical 400-800 N applied axial shifting load (often expressed as a ratio of the applied axial shifting load F to the breakthrough load BTL, such known ratios are typically well in excess of 10:1, and perhaps even 20:1 or more), using ramp angles in the range of about 30 to 40 degrees. Even when “firm” shift points are desired, known approaches maximally limit the breakthrough load to no more than about 80-100 N, or roughly fifteen percent of the applied load (i.e, an F-to-BTL ratio of at least about 6.6:1), using maximum ramp angles of up to about 45 degrees.
To rotationally couple one gear to the shaft, a fork applies the axial shifting load to the sleeve to thereby move the sleeve axially relative to the hub toward the gear. The sleeve then operates through the detent to apply a small fraction of the applied axial shifting load F (up to the detent's breakthrough load BTL) to each strut, thereby moving each strut axially toward the gear. The struts, in turn, axially engage a respective baulking or blocking ring, disposed between the hub and the gear and rotatable with the hub, toward a clutch ring that is itself rotationally coupled to the gear. Ultimately, the struts cooperatively urge a conical friction surface on the blocking ring into engagement with a complementary friction surface or “cone” defined on the clutch ring. The resulting frictional engagement between the blocking ring and the clutch ring generates a cone torque that rotationally accelerates or decelerates the clutch ring and its coupled gear relative to the blocking ring and, hence, reduce the rotational speed differential between the hub and the gear.
Ultimately, the blocking ring frictionally bears against the clutch ring sufficiently to stop relative rotation between the sleeve and the gear. The continued application of the axial shifting load to the sleeve then overcomes the detent, and the sleeve moves farther toward the gear as the unloaded strut likewise ceases to axially bear against the blocking ring, with leading-edge chamfers on the sleeve's spline teeth engaging opposing entrance-edge chamfers on a further set of peripheral teeth defined on the blocking ring. After the mating chamfers of the sleeve and the clutch ring cooperate to “clock” the sleeve's spline teeth into angular registration with the blocking ring's peripheral teeth, the sleeve moves even farther and its leading-edge chamfers engage opposing entrance-edge chamfers on a set of peripheral teeth defined on the clutch ring. After this second set of mating chamfers cooperate to clock the sleeve's spline into angular registration with the clutch ring's peripheral teeth, the sleeve moves into “full engagement” with the clutch ring to thereby rotationally “lock up” the selected gear with the shaft.
The foregoing known strut-type synchronizers typical achieve lockup synchronization times in the range of perhaps about 200-300 msec for a typical manual transmission, from the initial movement of the sleeve from its neutral position about the hub, to the point at which the opposed chamfers of the sleeve spline and the clutch ring first begin to mesh (it will be appreciated that an additional time of perhaps about 100 msec is still required to axially displace the sleeve into “full engagement” with the clutch ring, as described above).
In another known strut-type synchronizer, as disclosed in U.S. Pat. No. 5,085,303, a pair of radially-nested friction surfaces or “cones,” defined on either side of a “middle cone ring” rotating with the gear and respectively engaging complementary friction surfaces on the blocking ring and an “inner cone ring” rotating with the synchronizer's mainshaft, cooperate in response to the axial movement of the sleeve and struts to generate a relatively-increased cone torque, even when using a relatively-reduced applied axial shifting load or a relatively-reduced detent breakthrough load. Unfortunately, such dual cone synchronizers necessarily feature both an increased parts count, including discrete parts for the blocking ring, the two cone rings, and the clutching ring, and a relatively-increased overall axial synchronizer dimension, in order to achieve the relative increase in generated cone torque. Further, such “dual-cone” synchronizers are susceptible to torque-generation losses as the multiple cone rings link together, and due to tolerance stack-up, such that the best estimate for the resulting cone torque is the square root of the sum of the squares of the cone torque generated due to engagement of each individual cone.